HFM/EXED

High Magnetic Field Facility for Neutron Scattering

The HFM/EXED proposals can be submitted at any time. Only soft deadlines, after which the proposals are sent to the referees, are announced. The next soft deadline is June 1 2017.

Modes of operation available in this call

-TOF Diffraction

-TOF Low-Q

-Direct TOF Spectroscopy

(for details, see the instrument section below and Refs. [2-3])

Writing a proposal

-All the proposals have to be discussed with the local contact and contain supplementary information on bulk sample characterization in high fields, previous neutron measurements, evidences of Q-range accessibility using EXEQ-calculator and detailed measurement plan (for details, see a short Presentation).

Experiment planning

- Diffraction: A web-based interface for calculating the instrument range and finding the optimal sample orientation is available at EXEQ. A stand-alone version is available for download. It contains a full featured WinPython 2.7.6.4 necessary to run EXEQ as well as inEXEQ, see below.

- Spectroscopy: A stand-alone inEXEQ softwarefor calculating the instrument E,Q-range and finding the optimal sample orientation is available for download. Mantid users can install a nightly built Mantid version and use DGS Planner - an interface for plotting expected coverage for direct geometry time-of-flight instruments.

Preparing your experiment

-Note, since no sample rotation is available at the moment, the sample has to be properly oriented and fixed on a sample holder prior to the experiment. Information about the sample space geometry and dimensions are available in the download area. Limited capabilities for checking the sample orientation can be provided on E4 2-axis diffractometer.

-Always check the news stream below to have the latest updates.

News:

- May 12, 2017 - both inEXEQand Mantid DGS Planner are available for calculating the E,Q coverage for EXED in direct spectrometer mode

-March 27, 2017 - a new release of EXEQ is available for download. Version 0.16 reflects the geometry of the instrument after the installation of the new detectors, chopper and vacuum chamber. Please use the new version from now on, as the accessible scattering angles on EXED have changed.

-March 27, 2017 - 26 T is back! The new resistive-coil has been installed and successfully tested.

-March 2017 - First inelastic measurements of standard samples.

-Oct 2016 - Installation of the inelastic components is complete

Facility description

High Magnetic Field Facility for Neutron Scattering consists of two main components: the High Field Magnet (HFM) and the Extreme Environment Diffractometer (EXED). The former is a dedicated 26 T hybrid magnet, built by the HZB in collaboration with the National High Magnetic Field Laboratory (Florida, US). The latter is a time-of-flight instrument optimized for neutron scattering in restricted angular geometry of the magnet.

The High Field Magnet

The HFM is a "first of its kind" hybrid magnet system reaching fields as high as 26 T, making it by far the strongest continuous field available for neutron scattering experiments worldwide (see Fig. 1 below) [1]. The HFM utilizes Series Connected Hybrid System Technology where water cooled resistive insert coils are mounted in the room temperature bore of a superconducting cable-in-conduit solenoid. Operation of the magnet system requires a dedicated technical infrastructure consisting of high-pressure water cooling for the resistive coil, 4 K Helium refrigerator for cooling of the superconducting coil and 20 kA DC power supply. More information on the magnet can be found in [1] and references therein and under the following link: HFM.

As the field is horizontal, a special feature of the HFM is 30° degrees conical openings at both ends of the resistive insert envisaged for neutron-scattering access. Rotating the magnet by a maximum of 12° with respect to the incoming beam results in 2θmax≈27° in forward scattering.

Sample Environment

A dedicated 3He horizontal cryostat allows high-field experiments to be combined with temperatures down to about 0.6 K for samples with cross section <1.5x1.5 cm2.

Another cryostat enables sample rotation around vertical axis by +/- 90 deg and temperatures down to about 1.5 K (4He) for samples with cross section <1.5x1.5 cm2.

A special dilution fridge (0.1 K) is under construction in collaboration with Uni. Birmingham.

The instrument

Time-of-flight instrument EXED (Fig. 2) is equipped with a multispectral extraction system as a part of about 70 m long supermirror guide [2]. As a result, neutrons from both thermal and cold moderators with a wavelength range from 0.7 to 15 Å are available for experiments. The lower wavelength limit is given by the kink in the guide that blocks the direct view of the source and provides a sharp cut-off. Apart from the kink, the guide is essentially straight (100x60 mm2 (HxW)) and ends with a two-channel focusing section that compresses the beam spatially in both directions. For applications requiring low beam divergence, the focusing end can be replaced by a 6 m long pin-hole collimation section with variable apertures.

Flexibility of the instrument is ensured by three alternative systems that are available to create neutron pulses: a curved Fermi chopper for very high resolution (Δt ~ 6 μs), a straight Fermi chopper for high resolution (Δt ~ 15 μs) and a double disc chopper for medium to low resolution (from 115 μs up to >5000 μs). A number of single disc choppers located downstream prevents frame overlap and defines the bandwidth of interest. Due to the chopper system one can operate the instrument from narrow (~ 0.6 Å) to wide (~ 14.4 Å) wavelength band mode centered at the region of interest, and easily trade resolution for intensity. For inelastic applications there is a double disc chopper (Δt ~ 15 μs) in front of the sample. When running, it picks up a single wavelength out of the wavelength spectrum.

The secondary instrument is equipped with position-sensitive 3He detector tubes combined in 6 detector panels and positioned in forward (FWD) and backward (BWD) scattering to reflect the geometry of the magnet (Fig. 2). The backscattering panels are stationary, while the forward ones rotate together with the magnet.

Operation Modes of HFM/EXED

In order to enable a broad range of scientific applications using a unique combination of neutron scattering and high magnetic fields, EXED combines several scattering techniques in one instrument. The operation modes of the instrument are described below.

Diffraction modeis used for studying single crystalline and powder samples in high magnetic fields [2].The accessible Q-range is from 0.1 up to 3 Å-1 in FWD (mainly in direction perpendicular to the field) and from 1 up to 12 Å-1 in BWD (mainly along the field). The precise Q-maps can be obtained by means of EXEQ-software.

Low-Qmode enables studies of mesoscopic entities in high magnetic fields such as e.g. vortex state in type-two superconductors [2]. Its momentum transfer range extends down to 10-2 Å achieved by a 6m-long collimation section which replaces the focusing guide.

Spectroscopy mode has been built to study magnetic fluctuations and excitations as function of high magnetic field, and is best suited for non-dispersive modes along the field (e.g. 1D- and 2D-sytems, isotropic systems with respect to the field, etc). EXED enables energy-resolved measurements over a limited Q-range < 3.25/λ (Å-1) for the incoming energies below 25 meV (>1.8 Å) and energy resolution of a few percent [3]. The precise (Q,E)-maps can be obtained by means of inEXEQ software for inelastic mode. Mantid users can benefit from DGS Planner from nightly biult Mantid.

Experiment planning

We provide a web-based interface for calculating the instrument Q-range in elastic mode and finding the optimal sample orientation. It is available at:

Interactive versions of both EXEQ and inelastic inEXEQ are available for download. The code is written in Python 2.7.x, and installation notes are included in the archive. The installation package includes a full featured WinPython 2.7.6.4 necessary to run (in)EXEQ. Please keep in mind that this is a beta-release and correct operation of the software may not always be guaranteed. Also, this is free software, and therefore all the code is available for reading. The current version of EXEQ available for download is 0.16, which is the same version as the one accessible via the web interface. This new version describes the instrument geometry correctly, including the changes related to the introduction of the inelastic mode on EXED. It is a first release of inEXEQ sowtware. Its web-base interface will be provided in future.

Please note that the code has not been optimised for speed as yet. If the web-based version of the code (which performs the calculations on the server side) is not performing up to your expectations, you may want to try downloading the interactive version and running it on your own computer. The downloadable version writes processed detector definitions to hard disk, therefore its start-up speed should improve after the first time it has been used. The parameter files, where the sample definition and orientation are saved, can be transferred between the web interface and the stand-alone version of EXEQ.